Pemphigus foliaceus (PF) is an autoimmune skin disease that has been recently characterized as a simple model for other organ-specific, antibody-mediated autoimmune diseases. An experimental passive transfer animal model is available for this disease and the resulting lesions are rapidly produced (less than 24h) and easily monitored. The investigators have recently shown that monovalent Fab fragments of the autoantibodies (AAb) are pathogenic and produce the disease faster and more efficiently than the intact AAb proving the primary lesion is the direct result of blocking the function of the antigen (Ag) by the AAb. The PF Ag is thought to be a 160 kD desmosomal core protein, desmoglein I (DG-1). The investigators have recently reported the presence of an insoluble trypsin resistant Ag in the cornified cell envelope fraction of human epidermis. An immunoreactive soluble 50 KD papain fragment of this PF Ag (pf-PF) could be released into solution and specifically immunoprecipitated by PF sera. Also, the investigators obtained another 45 KD fragment (tf-PF) by trypsin treatment of viable epidermal cells, and the investigators could partially purify it by concanavalin A affinity chromatography and immunoprecipitation. These findings suggested the presence of three different pools of PF Ag: a soluble pool from which PF Ag was initially immunoprecipitated by other investigators, a cell-surface trypsin-sensitive pool, and a cornified cell envelope-associated trypsin resistant pool. The aims of this grant proposal are to purify these fragments, study their relation to DG-I or the native PF Ag, determine differences between these pools of PF Ag, and study the epitopes of the Ag that are detected by PF autoantibodies and are responsible for the pathology in this disease. DG-I, pf-PF, and tf-PF will be purified and characterized chemically and immunologically. For the chemical characterization, amino acid and carbohydrate analysis, peptide mapping, and partial sequencing will be done. For immunological studies, polyclonal and monoclonal antibodies will be raised and their immunochemical and possible pathogenic potentials studied. Monoclonal antibodies will be used for epitope mapping by competitive studies with the PF AAb. Antibodies will be also used for preparation of pure PF Ag or its proteolytic fragments for further characterization and to study their potential pathogenic and therapeutic effects. To assure sufficient supply of PF Ag and to help in the characterization of the immunoreactive epitopes, cDNA will be purified from a keratinocyte cDNA library, fusion protein will be tested for immunoreactivity, and probes will be made to search for PF Ag mRNA and full length PF cDNA. Finally, immunoreactivity of the native PF antigen or its fragments will be compared to that of the antigen expressed in vitro from full or partial length cDNAs. The applicants believe that these studies will likely provide important basic information about the Ag-AAb interactions that initiate the pathology in this autoimmune disease. This information should pave the way for consideration of specific therapeutic intervention in PF as a model for other antibody-mediated autoimmune disorders, and will aid significantly in understanding differentiation and cell adhesion in the epidermis.